ALBERT

Description: Although sea-ice extent in the Bellingshausen-Amundsen (BA) seas sector of the Antarctic has shown significant decline over several decades, there is not enough data to draw any conclusion on sea-ice thickness and its change for the BA sector, or for the entire Southern Ocean. This paper presents our results of snow and ice thickness distributions from the SIMBA 2007 experiment in the Bellingshausen Sea, using four different methods (ASPeCt ship observations, downward-looking camera imaging, ship-based electromagnetic induction (EM) sounding, and in situ measurements using ice drills). A snow freeboard and ice thickness model generated from in situ measurements was then applied to contemporaneous ICESat (satellite laser altimetry) measured freeboard to derive ice thickness at the ICESat footprint scale. Errors from in situ measurements and from ICESat freeboard estimations were incorporated into the model, so a thorough evaluation of the model and uncertainty of the ice thickness estimation from ICESat are possible. Our results indicate that ICESat derived snow freeboard and ice thickness distributions (asymmetrical unimodal tailing to right) for first-year ice (0.29 ± 0.14 m for mean snow freeboard and 1.06 ± 0.40 m for mean ice thickness), multi-year ice (0.48 ± 0.26 and 1.59 ± 0.75 m, respectively), and all ice together (0.42 ± 0.24 and 1.38 ± 0.70 m, respectively) for the study area seem reasonable compared with those values from the in situ measurements, ASPeCt observations, and EM measurements. The EM measurements can act as an appropriate supplement for ASPeCt observations taken hourly from the ship's bridge and provide reasonable ice and snow distributions under homogeneous ice conditions. Our proposed approaches: (1) of using empirical equations relating snow freeboard to ice thickness based on in situ measurements and (2) of using isostatic equations that replace snow depth with snow freeboard (or empirical equations that convert freeboard to snow depth), are efficient and important ways to derive ice thickness from ICESat altimetry at the footprint scale for Antarctic sea ice. Spatial and temporal snow and ice thickness from satellite altimetry for the BA sector and for the entire Southern Ocean is therefore possible.

Description: Three ice type regimes at Ice Station Belgica (ISB), during the 2007 International Polar Year SIMBA (Sea Ice Mass Balance in Antarctica) expedition, were characterized and assessed for elevation, snow depth, ice freeboard and thickness. Analyses of the probability distribution functions showed great potential for satellite-based altimetry for estimating ice thickness. In question is the required altimeter sampling density for reasonably accurate estimation of snow surface elevation given inherent spatial averaging. This study assesses an effort to determine the number of laser altimeter 'hits' of the ISB floe, as a representative Antarctic floe of mixed first- and multi-year ice types, for the purpose of statistically recreating the in situ-determined ice-thickness and snow depth distribution based on the fractional coverage of each ice type. Estimates of the fractional coverage and spatial distribution of the ice types, referred to as ice 'towns', for the 5 km**2 floe were assessed by in situ mapping and photo-visual documentation. Simulated ICESat altimeter tracks, with spot size ~70 m and spacing ~170 m, sampled the floe's towns, generating a buoyancy-derived ice thickness distribution. 115 altimeter hits were required to statistically recreate the regional thickness mean and distribution for a three-town assemblage of mixed first- and multi-year ice, and 85 hits for a two-town assemblage of first-year ice only: equivalent to 19.5 and 14.5 km respectively of continuous altimeter track over a floe region of similar structure. Results have significant implications toward model development of sea-ice sampling performance of the ICESat laser altimeter record as well as maximizing sampling characteristics of satellite/airborne laser and radar altimetry missions for sea-ice thickness.

Description: Ice Station Belgica was commenced in late winter 2007 in the Bellingshausen Sea as part of Sea Ice Mass Balance in Antarctica (SIMBA), an IPY 2007 cruise on the research vessel N.B. Palmer. A primary objective was to build on the work of previous Antarctic drift station experiments to geophysically characterize sea ice in terms of thickness, surface and ice bottom morphology, and ultimately area-unitized mass. A 24 day drift station was established at approximately 70°S and 93°W in mixed first-year and multi-year ice with three geophysical study sites selected on a 5 km**2 floe. A comprehensive time series assessment of elevation-surveyed transects ranging from 100 m to 300 m in length included snow surface elevation, snow depth, electromagnetic (EM) profiling, and direct drilling for ice draft and ice freeboard. Additional work included a snow surface morphology characterization of a 100 m x 300 m area between the primary time series EM transects. Correlation of EM ice thicknesses with collocated drilled ice thickness yielded equations for the correction of EM underestimation of thick deformed ice, particularly at pressure ridges. Mean ice thickness from corrected EM was compared to isostatic ice thickness calculated from surface elevation, snow depth, ice freeboard and respective snow, slush, ice, and sea water densities. Results were consistent, with mean ice thicknesses for multi-year ice of 2.35 m, 2.34 m, and 2.41 m, with similar variance, for corrected EM, drilling, and buoyancy methods respectively. Additionally, a mean ice thickness of 2.31 m was calculated from ASPeCt observations of the ice field associated with the floe, using the method incorporating mean sail heights and fractional coverage of surface deformities or ridging. Temporal series assessment of ice freeboard indicated a slightly negative mean ice freeboard ( 〈 0.04 m), with clear evidence of new snow-ice formation from the freezing of slush. The three distinct snow and ice regions assessed on the Belgica floe had mean corrected EM ice thickness of 0.52 ± 0.04 m (± 1 std. deviation), 0.92 ± 0.17 m, and 2.35 ± 1.37 m, and mean snow depths of 0.08 ± 0.03 m, 0.36 ± 0.09 m, and 0.68 ± 0.31 m respectively. Each ice type represented a sizable fraction of the floe's total area (~ 20%, 40%, and 40% respectively from visual estimates) reflecting a complex dynamic and thermodynamic history of formation, as well as the difficulty in characterizing even a single floe by a single class or mean value for thickness and snow depth. Implications of these results are discussed with regards to the resolution of satellite-based altimetry and snow depth products and efforts to generate and validate satellite sea ice and snow thickness products.

Description: The Sea Ice Mass Balance in the Antarctic (SIMBA) experiment was conducted from the RVIB N.B. Palmer in September and October 2007 in the Bellingshausen Sea in an area recently experiencing considerable changes in both climate and sea ice cover. Snow and ice properties were observed at 3 short-term stations and a 27-day drift station (Ice Station Belgica, ISB) during the winter-spring transition. Repeat measurements were performed on sea ice and snow cover at 5 ISB sites, each having different physical characteristics, with mean ice (snow) thicknesses varying from 0.6 m (0.1 m) to 2.3 m (0.7 m). Ice cores retrieved every five days from 2 sites and measured for physical, biological, and chemical properties. Three ice mass-balance buoys (IMBs) provided continuous records of snow and ice thickness and temperature. Meteorological conditions changed from warm fronts with high winds and precipitation followed by cold and calm periods through four cycles during ISB. The snow cover regulated temperature flux and controlled the physical regime in which sea ice morphology changed. Level thin ice areas had little snow accumulation and experienced greater thermal fluctuations resulting in brine salinity and volume changes, and winter maximum thermodynamic growth of ~0.6 m in this region. Flooding and snow-ice formation occurred during cold spells in ice and snow of intermediate thickness. In contrast, little snow-ice formed in flooded areas with thicker ice and snow cover, instead nearly isothermal, highly permeable ice persisted. In spring, short-lived cold air episodes did not effectively penetrate the sea ice nor overcome the effect of ocean heat flux, thus favoring net ice thinning from bottom melt over ice thickening from snow-ice growth, in all cases. These warm ice conditions were consistent with regional remote sensing observations of earlier ice breakup and a shorter sea ice season, more recently observed in the Bellingshausen Sea.